This invention relates to an improved lubricating oil composition for internal combustion engines. Specifically, the invention relates to a lubricating oil composition containing an effective supplemental rust inhibitor (SRI).
Fully-formulated engine lubricants contain overbased detergents which impede engine rust formation by neutralizing the acidic components that are entrapped in the engine sump during fuel combustion. If a lubricant provides insufficient rust protection, more overbased detergent can be added to increase the neutralization reaction rate and thereby decrease the steady-state concentration of acidic components present in the oil. However, cost/performance restraints or sulfated ash restrictions limit the feasibility of this approach. When this occurs, anti-rust "fixes" must be employed or the additive package must be substantially altered.
Anti-rust "fixes" are usually non-ionic surfactants which can be used at relatively low concentrations. Although these materials generally have little or no anti-rust activity by themselves, when they are combined with overbased detergents rust inhibition is sometimes greatly improved. Unfortunately, there are few recognized supplemental rust inhibitors. Three such known SRI's which have been used either commercially or in experimental oils are Plexol 305, Pluronic L-101 and Tetronic 1501. Plexol 305 is an ethoxylated isooctylphenol and is available commercially from Rohm & Haas. Pluronic L-101 is a block polymer based on ethylene oxide and propylene oxide and is available from BASF-Wyandotte. Tetronic 1501 is a tetrapoly(oxyethylene)poly(oxypropylene) derivative of ethylene diamine which is available from BASF-Wyandotte.
Experience has shown that Pluronic L-101 has caused abnormal sludging problems in the field while the use of Tetronic 1501 can result in eventual precipitation of lubricant additive components. These materials are claimed as SRI's in U.S. Pat. Nos. 3,509,052 and 3,928,219, which are incorporated herein by reference.
Although Plexol 305 does not cause sludging problems or precipitation of lubricant additive components, unfortunately it has only modest anti-rust activity. Accordingly, it is an object of this invention to provide an additive which not only overcomes the sludging problems and precipitation problems of known SRI's but which also has good anti-rust activity.